The present invention relates to a numerical control system, and more particularly to a numerical control system which is suitably applied to a numerically controlled machine tool wherein a tool is moved or rotated in the direction of the three X, Y and Z axes (cartesian coordinate system) and the directions of the two of B and C axis (spherical coordinate system) so as to subject a workpiece to three-dimensional machining.
In a machine tool such as a machining center, a tool or a table is rotated in the vertical rotational direction (B-axial direction) and the horizontal rotational direction (C-axial direction) so as to control the tool axis direction of the tool relative to a workpiece or the table, and it is also moved in the directions of the three of X, Y and Z axes so as to subject the workpiece to a desired machining operation.
For example, in forming a hole in the workpiece under 5-axis control, first of all, a tool holder is positioned by moving it in the X-, Y- and Z-axial directions unitarily with the tool, while the tool axis direction of the tool and the direction of the hole to be machined are brought into agreement by rotating the tool in the vertical rotational direction and the horizontal rotational direction. Thereafter, whilst maintaining the tool axis direction, the tool holder is moved toward the workpiece by simultaneous 3-axis control of the X-, Y- and Z-axes. The machining of the hole is started, and the drilling operation is performed down to a predetermined depth. Lastly, the tool is drawn out in the opposite direction to complete the drilling operation. This system has hitherto been proposed.
In a machine tool such as is used in a machining center which conducts machining with the tool inclined relative to the tool holder in this manner, there occurs (1) a case where it is desired to manually change the tool axis direction so as to establish a proper machining condition or so as to sever the interference between the tool and the workpiece in the course of performing the three-dimensional machining by moving the tool in the directions of the three X, Y and Z axes, or (2) a case where it is desired to set the tool relative to the workpiece by manually changing the tool axis direction without changing the relative position between the nose of the tool and the workpiece.
With the prior-art system, however, when the tool is merely rotated in the B- and C-axial directions to change the tool axis direction, the nose of the tool moves, so that the relative positional relation between the nose of the tool and the workpiece changes. In other words, when the tool axis is moved during three-dimensional machining, machining operations as programmed cannot be performed thenceforth on account of the movement of the nose of the tool. Moreover, the manual setting of the tool relative to the workpiece in the prior-art system must conform with the procedure in which the nose of the tool is first located at a predetermined position relative to the workpiece, the tool is subsequently rotated to change the tool axis direction and the nose of the tool is thereafter located again at the point where it lay prior to rotation. The setting operation therefore is very troublesome.